1. Field of the Invention
The invention relates to a system for flushing at least one internal space of an objective.
More specifically the invention relates to a system for flushing at least one internal space of a microlithographic projection exposure objective for manufacturing semiconductors.
2. Description of the Invention
It is generally known to seal off objectives, in particular exposure projection objectives for manufacturing of semiconductors, from the surroundings and to flush the internal space(s) of the objective with the aid of a flushing gas in order to avoid contamination from outside, the operation being performed with a slight overpressure.
Because of the chemical stability of inert gasses, it has already been proposed to carry out flushing with the aid of such a gas.
With regard to the general prior art, reference may be made in this context to U.S. Pat. No. 5,157,555, in which it has been proposed to undertake a correction of spherical aberrations by means of a variable air gap between neighboring surfaces.
U.S. Pat. No. 6,252,648 B1 describes an exposure apparatus in microlithography, and an exposure method, a closed space being introduced in an objective in order to avoid contamination and to clean a mixture or an inert gas and oxygen.
U.S. Pat. No. 4,871,237 has proposed improving the optical imaging accuracy of an objective by changing the barometric pressure of an internal space of an objective. Various gasses and gas mixtures are proposed as medium for the internal space in order to change the refractive index in the gas mixture.
Flushing with the aid of an inert gas entails comparatively high costs which, although entirely acceptable during the operation of an objective, entail a substantial cost outlay in the phase of adjusting and mounting. If it is now desired to operate an objective in this phase of mounting and adjustment with the aid of the very cost-effective flushing gas of air, its imaging is calculated for flushing with air. If, in its final proper use, however, the objective is to be flushed with the very much more expensive inert gas, the problem arises that these gasses have a refractive index which deviates substantially from the refractive index of air. This results in aberrations which would lead to malfunctioning of the objective.
It is therefore the object of the present invention to create a system for flushing at least one internal space of an objective, on the one hand the use of inert gasses for flushing being rendered possible, but on the other hand no changed refractive index being introduced.
This object is achieved according to the invention by means of a system for flushing at least one closed internal space of an objective, the flushing being performed by mixing at least two inert gasses in such a way that the refractive index resulting therefrom corresponds at least approximately to the refractive index of air.
According to the invention, inert gasses are now intermixed in such a way that the refractive index resulting therefrom corresponds to the refractive index of air. The result of this is that the objectives can firstly be operated with the very favorable flushing gas of air and being adjusted and mounted. Not until the final adjustment and/or the final proper use of the objectives in situ is the change then made to flushing with an inert gas mixture. In this case, there is the advantage of chemical stability over a long operating period. Aberrations which lead to malfunctioning of the objective are reliably avoided by the adaptation of the refractive index.
Experiments have shown in practice that the object set can be achieved very effectively in an economical way when, in the case of the use of two inert gasses, use is made of nitrogen and helium as the gasses, nitrogen constituting the main constituent with 95 to 99.5% by volume, preferably even 98.8%, and this mixture can be used to create in conjunction with helium a refractive index which largely corresponds to that of air.
Of course, yet further inert gas mixtures are possible with other gas compositions within the scope of the invention.
By means or an appropriate adaptation or mixing ratio, it is also possible if necessary specifically to set refractive index changes for the purpose of correcting aberrations which occur.
This means that, if necessary, objectives already used in practice can also be improved by means of this gas mixture.